Modeling dynamic systems, including fluid dynamic systems, using computers, particularly high-speed digital computers, is a well known and cost efficient way of predicting system performance for both steady state and transient conditions without having to physically construct and test an actual system. A benefit to computer modeling is that the effect on performance of changes in system structure and composition can be easily assessed, thereby leading to optimization of the system design prior to construction of a commercial prototype.
Known modeling programs generally use a “multi-cell” approach, where the structure to be modeled is divided into a plurality of discrete volume units (cells). Typically, the computer is used to compute thermophysical values of the fraction of the system within the cell, such as, e.g., mass, momentum, and energy values, as well as associated fluid system design parameters such as density, pressure, velocity, and temperature, by solving the conservation equations governing the transport of state value units to or from the neighboring cells. For a fixed geometric system model using Cartesian coordinates, and absent a system boundary, each cell would have six cell neighbors positioned adjacent the six faces of the cube-shaped cell. An example of a computational fluid dynamics (“CFD”) modeling program is the MoSES Program (available from Convergent Thinking LLC, Madison, Wis.).
There are two types of boundary fitted grids. The more conventional type of boundary fitted grid morphs the cells near the boundary to conform to the shape of the geometry, e.g., a six-sided cell near a boundary would not necessarily be a perfect cube. The other method is commonly called a “cut-cell” method. In a typical boundary fitted grid, moving surfaces are handled by further morphing the near wall cells. For “cut-cell” methods, the underlying cells do not move with the boundary; instead, the boundary motion simply results in a new series of cuts to the underlying grid. The method described here offers a way to properly transport thermophysical properties during the moving process for a “cut-cell” grid.